The invention relates to a method of manufacturing optical fibers. In the method, glass layers are deposited on the inside of a glass tube which is heated to a temperature between 1100.degree. and 1300.degree. C. The glass layers are deposited by passing a reactive gas mixture through the tube at a pressure between 0.13 and 13.33 kPa, and reciprocating a plasma in the interior of the tube. After a sufficient number of glass layers have been deposited, the tube is collapsed to form a solid preform. The preform is then drawn into an optical fiber. Glass is to be understood to mean in this connection also synthetic glass and amorphous quartz obtained from quartz crystals by fusion (i.e. fused silica and quartz glass), when doped or undoped.
The manufacture of optical fibers by this method is disclosed, inter alia, in U.S. Pat. Nos. RE 30,635 and 4,314,833. In the art, this method is known as the "non-isothermal plasma C.V.D." process (nonisothermal P.C.V.D.). In this method, glass layers are deposited directly from the gaseous phase on the inner wall of the glass tube (a heterogeneous reaction). The formation of glass soot in the gaseous phase is avoided (see in this connection, in particular, U.S. Pat. No. 4,314,833).
In principle it is not necessary in this method to rotate the glass tube so as to ensure a rotationally symmetrical glass deposition. In practice, the diffusion in the gaseous phase of charged particles formed in the plasma (ions, excited atoms and the like) to the glass wall does not seem to be biased by gravity. This in contrast with a so-called homogeneous deposition process in which glass particles are formed in the gaseous phase; the deposition of the glass particles is indeed biased by gravity.
In methods in which glass particles are formed in the gaseous phase and are deposited on the interior of a tube, the tube is rotated continuously at a rather high speed so as to ensure a rotationally symmetrical glass deposition. (See, for example, U.S. Pat. No. 4,217,027, which describes the M.C.V.D. process, in which the speed of rotation is 100 rpm). The gas pressure in these deposition processes usually is equal to or larger than atmospheric pressure. Therefore no particularly high requirements need be imposed upon the rotation couplings with respect to their gastightness.
In industrial production of optical fibers according to the P.C.V.D. process, it sometimes appears that the deposited glass layers are sometimes not rotationally symmetric. This is detrimental because nonsymmetrical deposition of the glass layers may lead to a reduction of the expected bandwidth.
The nonsymmetrical deposition has been found to have various causes which each individually or in combination can detrimentally influence the optical properties of the manufactured optical fiber. If during the movement of the resonant cavity for generating the plasma the glass tube is not in the center of the resonant cavity throughout its length, a rotationally nonsymmetrical plasma distribution will occur in the tube. The glass tube may have imperfections which may also result in a nonuniform deposition of the glass layers. These imperfections may be that the tube is not completely straight, or the tube does not have the same outside or inside diameter throughout its length. Furthermore, imperfections in the furnace may lead to a nonuniform heating.
Preventing or reducing the disadvantages of a rotationally nonsymmetrical deposition of the glass layers by continuous rotation of the glass tube during the deposition as used, for example, in the above-mentioned M.C.V.D. process, meets with a number of disadvantages in the present P.C.V.D. process. These disadvantages are related to the differences in performing the processes.
The M.C.V.D. process is usually carried out at pressures about equal to atmospheric pressure or greater than atmospheric pressure. Leakage in the rotation coupling used does not result in leaking-in of the ambient atmosphere into the glass tube. In contrast, the P.C.V.D. process is carried out at lower than atmospheric pressure. If the rotational coupling is not completely sealed from the atmosphere, the ambient atmosphere is drawn into the tube with all the resulting detrimental disadvantages. These disadvantages include, for example, an increase of the attenuation by incorporation of water in the glass layers which are formed in the tube. Detrition products originating from the rotating parts of the coupling can also be drawn into the tube and can also detrimentally influence the properties of the optical fiber.
An adequate rotational seal which can be used continuously without the described disadvantages is difficult to realize in practice. The consideration should be taken into account that already extremely small quantities of impurities (on the order of 0.1 ppm) may give rise to a noticeable increase in the attenuation (on the order of 1 db) in the optical fiber.